1. Technical Field
This invention generally relates to an optical projection system comprising a light source, a mask holder, a projection lens system. The projection system specifically relates to an optical projection system for photolithography used in producing microstructured devices such as integrated circuits or other semiconductor devices. During the fabrication of such devices photolithography transfers an image from an photographic mask to a resultant pattern on a semiconductor wafer. Such photolithography generally includes a light exposure process, in which a semiconductor wafer is exposed to light having information of a mask pattern. Optical projection systems are used to perform the light exposure process.
In general, the transferred mask patterns are very fine, so that optical projection systems are required to have a high resolution. The high resolution necessitates a large numerical aperture and a good correction of aberration of the optical projection system in the light exposure field.
2. Background Art
Projection lens systems used for photolithography consists of a lot of lenses, wherein the material of the lenses is very expensive. To reduce the number of needed lenses, lenses with aspherical surfaces are used.
For example some projection lens systems are proposed in the German patent application DE 198 18 444 A1 or DE 199 02 336 A1. The shown projection lens systems consists of 6 or 5 lens groups. The first, third and fifth lens group have positive refractive power. If the projection lens system consists of six lens groups, then the sixth lens group has also positive refractive power. The second and fourth lens groups have negative refractive power. To get a high resolution, in all shown examples the fourth and fifth lens groups comprises lenses with aspherical surfaces. The distance between a mask in front of the lenses of the projection lens system and a wafer behind the lenses of the projection lens system is between 1200 mm and 1500 mm. But only in some projection systems such a large track length is provided for the projection lens system.
The projection lens systems shows three bulges, or convex portions. The diameter of a bulge, i.e., convex portion, is defined by the maximum height of the principle rays, which is nearly the diameter of the used lenses. In the shown embodiments the diameter of the first bulge, i.e., convex portion, is smaller than the diameter of the third convex portion. If the projection lens system consists of six lens groups, only one bulge, i.e., convex portion, is established by the fifth and the sixth lens groups.
With increasing diameters of the needed lenses, the price of the projection lens system is getting up.
Further projection lens systems comprising aspherical surfaces are part of the patent application DE 199 422 81 A1.
It is an object of this invention to provide a further excellent optical projection lens system for photolithography with high numerical aperture and a good optical performance in respect of track length and cost effects.
A projection lens system of the invention comprises one lens with an aspherical surface. It consists of a first and a second bulge, wherein a first waist is arranged between the bulges. With a projection system comprising a second bulge, which is smaller than the first bulge, the number of needed lenses with a great diameter is reduced. To get such a design with a small second bulge at least one lens with an aspherical surface is needed.
Further the number of needed lenses can be reduced by taking a second lens group consisting of three lenses, especially three negative lenses.
Further it is helpful to have a projection lens system comprising both features, the small second bulge and the first waist consisting of three lenses.
An optical projection system of the invention comprises in a direction of the propagating ray a first lens group having positive refractive power and a second lens group having negative refractive power and establishing a first beam waist of minimal beam height. A third lens group having positive refractive power and a fourth group having negative refracting power, establishing a second beam waist. Subsequent to the second waist a fifth lens group can be divided into a first subgroup comprising an aperture stop and a second subgroup.
Two negative lenses are arranged nearby the aperture stop. Behind the first positive lens, which is arranged subsequent to the aperture stop a lens free distance follows. This lens free distance extends more than 10% of the track length of the fifth lens group or more than 4% of the track length of the projection lens system.
At least two lenses arranged behind the aperture plate comprises as aspherical surface. Further lenses comprising aspherical surfaces in all other groups will be helpful for correction of chromatic errors for such a projection lens system with such a high numerical aperture.
By taking a projection lens system with a third bulge, wherein the diameter of this bulge is at least 10% greater than the diameter of the second bulge, the cost of the projection lens system with a high numerical aperture can be reduced, because the great diameter of the last bulge is needed to get the high numerical aperture, but by taking a second bulge with a small diameter, it is possible to reduce the number of needed lenses with a great diameter, which are very expensive. So this is a good way to provide an excellent projection lens system at reduced cost.